1. The Field of the Invention
The present invention relates to methods for removing or reducing air pollutants in combustion flue gases. More particularly, the present invention relates to a method and system for the reduction of nitrogen and sulfur oxides emissions from carbonaceous fuel combustion flue gases in industrial and utility furnaces.
2. The Relevant Technology
The use of coal water slurry as a fuel is well known, but historically in the United States, with minor exceptions, the use of coal water slurry has been confined primarily to demonstration projects. The disadvantages of using coal water slurry in the past have been related to the energy required to evaporate the water in the slurry, and the relatively high price of preparing and transporting a high coal density (65% plus coal) coal water slurry fuel. However, with new coal processing techniques, waste coal fines from coal perparation plants, which have been impounded in ponds, may now be recovered very economically in a coal water slurry form. The recovered coal water slurry fuel in many cases will be less expensive than the processed coal sold from these preparation plants. It is estimated that two to four billion tons of such coal fines reserves are impounded in ponds throughout the United States. It would be very beneficial to use these low cost waste coal fines as a fuel and to do so in an environmentally acceptable manner.
In U.S. Pat. No. 3,589,314 to Tratz et al., a method and device for pressure spraying and burning a 60/40 weight ratio coal dust/water mixture is described. Complete combustion of the coal dust/water mixture in a furnace is accomplished by heating the mixture under pressure prior to spraying the mixture through a nozzle with radial or axial bores into a combustion chamber. Additionally, U.S. Pat. No. 4,444,126 to Forster describes an apparatus for combustion of a suspension of coal particles in water, in which combustion air preheated to 550.degree. C. (1022.degree. F.) is forced into a coal water slurry preheated to 100.degree. C. (212.degree. F.) in a portion of the burner, upstream of the burner flame. In U.S. Pat. No. 4,465,495 to Scheffee, a process is disclosed for making a high heating value coal water slurry fuel that can be injected directly into a furnace as a combustible fuel for the express purpose of replacing fuel oil in oil-fired boilers.
Furthermore, U.S. Pat. No. 5,380,342 to Leonard et al. describes a process of co-firing coal water slurry specifically in a gas fired boiler through gas burners. The furnace is preheated using natural gas and is then fired with approximately 60% of the heat input supplied by pulverized coal and 40% of the heat input supplied by natural gas. The gas is gradually shut off completely while coal water slurry is added such that the heat input ratio of pulverized coal to coal water slurry is 80/20. The process of this patent is specifically directed to a modified gas fired boiler wherein the burners are designed to fire three fuels: natural gas, coal water slurry, and pulverized coal.
While the above processes accomplish their intended purpose to facilitate the combustion of coal water slurry in fossil fuel boilers, they are not specifically designed to reduce the emissions of oxides of nitrogen and sulfur, which are acid rain precursors, to the atmosphere.
In pilot scale testing, an expensive to produce, high density (72% coal/28% water) coal water slurry fuel has been tested as a reburn fuel on a cyclone-fired test unit under a U. S. Department of Energy Research and Development program (DOE Report DOE/PC/190157-1). Under this test work, coal water slurry was fired through a reburning burner with some 40% stoichiometric air into a reburn zone. A NO.sub.x reduction of approximately 60% was achieved with at least 30 to 35% of the boiler heat input coming from the coal water slurry reburn fuel. With this approach, however, air addition to the burner creates an excessive reburn fuel requirement in order to reduce the air stoichiometry in the reburn zone to the desired level for good NO.sub.x reduction. Further, the more fuel that is fired in the reburn zone, the more the reburn fuel alters the furnace heat absorption profile. Still further, the reburn technique used was based on the use of a high cost, high density coal water slurry fuel. While this technique will reduce the emissions of oxides of nitrogen to the atmosphere, it does not reduce sulfur dioxide emissions and is not very cost effective or operationally beneficial.
Several U.S. Department of Energy, Clean Coal Technology (CCT) commercial demonstrations have been completed recently based on the partial oxidation of natural gas and fine pulverized coal for utility boiler in-furnace reduction of nitrogen oxides. These technologies are called "reburn" technologies. With reburn technology, coal supplied to burners for cyclone-fired, tangentially-fired and wall-fired boilers is combusted under normal excess air operating conditions at about 70% to 90% of the total heat input. Reburn fuel is then injected into the furnace in a reburn zone above the conventional burners either through burners or injectors. In the reburn zone, the overall stoichiometry of combustion is reduced from an excess air condition to an air deficient condition through addition of the reburn fuel. Above the furnace reburn zone, additional air called "overfire air" is added to once again create an excess air condition in the boiler, hence the name "reburn" was applied to this technology.
Various techniques for injecting alkali compounds into coal-fired boiler furnaces both through and above pulverized coal-fired burners, for the capture of sulfur oxides as alkali sulfites and sulfates is also known.
While the above mentioned reburn and alkali injection processes have been demonstrated for reducing the emissions of oxides of nitrogen and sulfur from fossil-fuel fired boilers, an improved process is needed that will allow for the use of a low cost coal water slurry fuel that when fired in a boiler will reduce atmospheric emissions of acid rain precursors, in the form of oxides of nitrogen and sulfur, and offer to electric and industrial utilities a very low cost option with minimal boiler thermal impacts, to meet the more stringent environmental regulations in the furture.